AN-5233 Datasheet, PDF (1/4 Pages) Fairchild Semiconductor – Consideration of Power MOSFETs

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AN-5233
Consideration of Power MOSFETs in Fast Charger Design
Abstract
The demand for battery capacitance up to 10,000 mAh in
mobile devices is steadily increasing as devices consume
more power and offering more functionality. Large battery
capacity provides longer battery run-time, but needs more
time to charge. This application note describes how to
decrease charge time and the impact of power MOSFET
selection, including a practical design example. The one
serial and three parallel (1S3P) 9,700 mAh Li-Ion battery is
used and 1C and 0.5C rates are set to measure charging and
discharging time, respectively. Products that require large
battery capacitance and fast charging time include tablet /
notebook docking stations, portable music players, and
commercial electronic Point-of-Sale (POS) systems.
Introduction
Capacity Rate(C-Rate)
In describing batteries, charge or discharge current is often
expressed as a C-rate. The C-rate means a discharge rate
relative to the capacity of a battery in an hour. A rate of 1C
means an entire 3000 mAh battery is discharged in one hour
at a discharge current of 3 A. A 0.5C rate indicates a
discharge current of 1.5 A. The battery charging time of
common Constant Current / Constant Voltage (CC/CV)
mode depends on C-rate of CC Mode.
Charging Voltage
Cell Connection: Serial vs. Parallel
Battery cells can be connected in series or parallel to
achieve high capacity. Connection of cells determines
battery pack voltage, C-rate, and charging time by constant
current (CC). Table 1 shows several advantages of serial
and parallel connection. The trend of mobile applications is
forward 1S and parallel connection because 1S achieves
lower system voltage, resulting in high DC-DC conversion
efficiency, and parallel increases battery capacity.
Table 1. Advantages of Serial and Parallel
Connection
Connection
Advantages
Serial
n
High-voltage output for high-power system
Short charging time
Low charging current
Parallel
n
Low-voltage output for low-power system
High charging current
Low voltage rated components
A typical charger design consists of bi-directional
MOSFETs to prevent inrush current and reverse current
blocking at the front-end, high-side, and low-side
MOSFETs (Q_HS, Q_LS) for step down and battery switch
(Q_BATT) to provide the charge and discharge path for the
battery pack, as illustrated in Figure 2.
Charging Current
Figure 1. Battery Charging Time Comparison by C-Rate
(9,700 mAh, 1S and 0.05C Cut-off Charging)
For the example Li-ion battery charging shown in Figure 1,
the 1C-rate charging for 9,700 mAh shortens charging time
by four (4) hours compared to the 0.2 C-rate case. For
designing a fast charger, a high C-rate charging is essential.
Figure 2. Charger Design Example for 1S3P Battery
The DC-DC buck converter for 1S battery charging is
chosen to cover a wide input range, up to 12 V_IN and 4.2 V
battery voltage with CC/CV Mode. It offers small form
factor with optimal passive elements at a high frequency.
The next section considers selecting power MOSFETs for
high efficiency and power density for fast charger design.
© 2013 Fairchild Semiconductor Corporation
Rev. 1.0.1 • 10/27/14
www.fairchildsemi.com